The present disclosure relates to voltage regulation and in particular to voltage reference circuitry having enhanced characteristics to variations in a beta parameter of the circuitry.
Unless otherwise indicated herein, the disclosure set forth in this section should not be construed as prior art to the claims in this application nor as admitted to be prior art by inclusion in this section.
Voltage reference sources are commonly used in integrated circuits. A bandgap voltage reference is a commonly used circuit block in analog designs which can provide a temperature independent and supply independent voltage reference. The voltage reference VREF in a bandgap circuit arises from two voltage components: VBE and VPTAT. The voltage VPTAT is a voltage that is proportional to the absolute temperature (proportional to absolute temperature). Circuits for generating VPTAT are known. The VPTAT voltage has a positive temperature coefficient (VPTAT increases with temperature), while VBE has a negative temperature coefficient (VBE decreases with temperature). Consequently, the resulting bandgap voltage VREF can be made insensitive to variations in temperature when VBE and VPTAT are properly combined.
A typical configuration of a circuit that provides VBE is shown in FIG. 6, where for example a vertical bipolar junction transistor (BJT) PNP transistor device Q and a current source 602 are connected in series between a voltage supply terminal 612 that is connected to a voltage source VDD and another voltage supply terminal 614 that is connected to ground potential GND. The base emitter voltage VBE, between the emitter terminal (E) of transistor Q and ground potential GND, is given by the relationship:
                                          V            BE                    =                      η            ⁢                                                  ⁢                          V              T                        ⁢            ln            ⁢                                          I                C                                            I                S                                                    ,                            Eqn        .                                  ⁢        1            where η is a technology dependent parameter,
      V    T    =      kT    q  is commonly referred to as the thermal voltage, IC is collector current, and IS is saturation current.
The collector current IC is given by the relationship:
                                          I            C                    =                                    (                              1                -                                  1                                      β                    +                    1                                                              )                        ⁢            I                          ,                            Eqn        .                                  ⁢        2            where I is an emitter current of the transistor Q, which in this circuit is provided by the current source 602. The parameter β is referred to as the common-emitter current gain, and is heavily process dependent. During semiconductor processing, the process conditions for fabricating a given lot of wafers typically are not identical to the process conditions for a subsequent lot of wafers. In fact, wafers in the same wafer boat will vary. Consequently, the β parameters for devices will vary from wafer to wafer. Variations up to ±30% in the value of β for devices on different wafers are not uncommon.
For process technologies where β>>1 and for a given constant emitter current I from the current source 602 in a specific design, the collector current IC will remain approximately equal to emitter current I despite variations in β because the
  1      β    +    1  term is small for large β's. However, for submicron processes (especially “deep” submicron processes such as 65 nM CMOS technology), β is small and may be on the order of β=1 or so. Consequently, devices from different wafers or different wafer lots may exhibit widely varying collector current IC characteristics due to its sensitivity to variations in β. Since VBE is a function of IC, bandgap voltage reference circuits based on a submicron process may exhibit wide variations in their respective VREF's.
A common VBE circuit that addresses the small β problem is the series cascade design shown in FIG. 7. Here, two BJT devices Q1, Q2 are connected in series. The voltage VBE is taken from transistor Q1 as shown in the figure. As can be appreciated, a base current IB2 in Q2 will compensate a base current IB1 in Q1. For the cascade circuit shown in FIG. 7, the collector current IC1 that flows through transistor Q1 is given by:
                              I                      C            ⁢                                                  ⁢            1                          =                              I            ⁡                          (                              1                -                                  1                                                            (                                              β                        +                        1                                            )                                        2                                                              )                                .                                    Eqn        .                                  ⁢        3            Since the β term in Eqn. 3 is squared, variations in β will have only a secondary effect on the collector current IC1 and so the sensitivity of IC1 to process variations is reduced; in other words, IC1≈I. This in turn results in bandgap voltage reference circuits whose voltage references VREF are less sensitive to process variation.
It will be appreciated that the circuit of FIG. 7 requires 2VBE headroom. Accordingly, in a voltage reference circuit that uses the circuit of FIG. 7 the headroom for the current source is computed as VDD-2VBE. Under common typical operating conditions, VBE may be on the order of 800 mV. Typically, VDD is 1.8 V and so the available voltage headroom for the current source is only about 0.2 V, which is generally insufficient for most designs of current sources and can impact the generation of accurate current flows.